In this letter, the influence of temporal and spatial diversities on the performance of ultra-wideband time-hopping pulse-position modulated multiple-access impulse radio (IR) systems is analyzed. We investigate how an antenna array can be used at the receiver to improve the bit-error rate (BER) performance and can cope with the effects of multiple-access interference of IR system in dense multipath environments. Analytical and simulation results show that the BER performance of the IR systems can be improved when the number of array elements is increased. The performance can be further improved by coherently adding more multipaths at the receiver